In caring for the critically ill patient, laboratory data should be available rapidly and, preferably, continuously. Blood electrolytes such as potassium, sodium, calcium and chloride are important elements. Of particular importance is the ability to measure ionized potassium continuously (in vivo) or at least rapidly (in vitro) to provide real-time data to the physician. The gradient of potassium across the cell membrane is the principal contributor to the membrane potential. Maintenance of this electrical potential is essential for normal function of all nervous and muscular tissue, including the conducting and contracting elements of the heart. The continuous or rapid measurement of potassium ion is important in intensive care, postcardiopulmonary by-pass, cardioplegia, the administration of digitalis and diuretics, acute myocardial infarction, renal failure and the treatment of burn patients and diabetic patients. A thin, catheter-mounted potassium electrode is not commercially available.
Many applications for ion selective electrodes have gone unsatisfied in the past for lack, not only of adequately selective sensing elements, but also because of difficulty in packaging the liquid elements of the measuring system. There are many examples but, selecting one of the above-mentioned conditions, there is a need to monitor potassium level in the blood of patients during and after major surgical procedures or during dialysis. It is a costly process to draw a blood sample every fifteen minutes or so and to have it analyzed in the hospital laboratory. More important, potassium level can change to critical value in less time than the time required to draw the sample, carry it to the laboratory, conduct the test, and report back to the operating team.
The situation could be greatly improved by provision of in vivo monitoring, but to do that requires a sensor that is small enough for insertion into a blood vessel. There has been no such sensor. Attempts to reduce the size of sensors necessarily involve reductions in the amount of electrolyte solution in the electrode. Heretofore, the result of such size reduction has been inaccuracy, and need for frequent recalibration due to drifting potentials.
Electrodes containing a liquid electrolyte can become a hazard to the patient should the sensing membrane, separating the electrolyte from the patient's blood, burst.
Aside from problems in miniaturization, such liquid filled electrodes cannot be sterilized by accepted sterilization procedures, such as ethylene oxide treatment, autoclaving and gamma radiation. These accepted sterilization procedures render such liquid filled electrodes inoperative by one or a combination of:
(1) physical damage to the ion sensitive membrane; PA1 (2) physical damage to other components of the electrode (sealing structure); PA1 (3) alteration of the chemical characteristics of the liquid electrolyte; PA1 (4) alteration of the ion selective properties of the sensing membrane. PA1 (1) a drastic increase in response time from ten seconds to milliseconds--an increase of about 100 times; PA1 (2) in the case of potassium and calcium electrodes, a 99 to 100 percent NERNST response; PA1 (3) the electrode can be repeatedly ethylene oxide or gamma radiation sterilized without effecting the stability or performance of the electrode; PA1 (4) the electrode may be stored wet or dry. PA1 (1) The sterile catheter combination electrode is placed in a venous blood vessel of the patient and the electrode lead connected to the patient channel of the analyzer; PA1 (2) The stat combination electrode is placed in a standard solution (which need not be sterile) of 4.0 milliequivalents K/L and the electrode lead is connected to the stat channel of the analyzer; PA1 (3) The reading on the stat channel is now adjusted to read 4.0; PA1 (4) A blood sample is drawn (venous blood) from the patient and transferred to a test tube; PA1 (5) The potassium content of the blood sample is measured with the calibrated stat electrode; PA1 (6) Finally, the patient channel is adjusted to read the same value as the stat channel.
Prior art efforts at miniturization have produced an electrode formed by cementing discs of ion selective membrane on the end of 3 mm outside diameter polyvinyl chloride tubes, filled with 3 molar KCl and fitted with a silver wire ( D. M. Band, J. Kratochvil and T. Treasure, Journal of Physics 265.5-6P, 1977). Units that small have not been commercially available. The problems that attend fastening a tiny disc of membrane material to the end of a tiny tube have not been solved. Even in 5 to 12 mm diameter sizes, electrodes of that design cost several hundreds of dollars.
It is necessary when using an ion selective electrode to use a reference electrode of steady potential in the measuring system. Like the selective electrode, the reference electrode includes a body of electrolyte and a half cell. Instead of a selective membrane, it includes a "salt bridge," but like the selective electrode, it has been large and cumbersome. In some applications requiring a miniaturized selective electrode, it matters little if the reference electrode is large, but in other applications there is a need for a miniaturized reference electrode. In still other applications there is need for a miniaturized reference device even if the selective electrode is not small.